This parametric OpenSCAD model allows you to generate a fruitfly trap for any jar that you have lying around.

Measure the outside diameter on the thread as shown in the picture. There will be an extra 0.5 mm of clearance added in the generated model to allow for printing tolerances. 

Open the OpenSCAD file and use the customizer to generate the model. For those who want more control, there are a bunch of constants in the source code to tweak the output even more.

Print it in the orientation in which it is generated (upside down). Strength-wise this is not a very demanding object so you could print it with a single perimeter if you like. Two if you are in doubt. Nozzle size or layer height don't matter much. 5 or 10% infill is ok.